a) Field of the Invention
The present invention relates to a control circuit which is applied to an open-loop control system and more particularly to a control circuit with frequency compensation, which can automatically perform frequency down-conversion when a turn-on time required by the control system is less than the minimum turn-on time.
b) Description of the Prior Art
As a light emitting diode is provided with advantages of having a high illumination efficiency, a long service time, not being damaged easily, consuming less electricity, being environmental friendly and having a small volume, it has been one of the most important light sources in recent years. In early years, as the light emitting diode does not have sufficient luminance, it was often used in an indicating lamp or a display panel. However, due to recent breakthrough in material science and technology, the luminance of the light emitting diode has already been improved significantly, especially that as a white-light light emitting diode emerges, existing conventional illuminating equipment has been gradually replaced by the light emitting diode.
On the other hand, as more and more applications of the light emitting diodes, a driving circuit to drive the light emitting diodes is becoming very important. The driving circuit can properly provide a stable power source to the light emitting diodes, allowing the light emitting diodes to illuminate stably. Whereas in order to reduce a cost and circuit space, many vendors will integrate the illumination driving circuit into an integrated circuit (IC) that by only using the illumination driving IC, the light emitting diodes can be effectively driven to illuminate.
However, when using the aforementioned illumination driving circuit, following issues and shortcomings actually exist to be improved.
None of the illumination driving circuit on a market can be applied to a condition of heavy line and light load, because at this time, a turn-on time required by the circuit is usually less than the minimum turn-on time; therefore, it results in that a current which passes through an illumination element, such as a light emitting diode, is too large that the light emitting diode will be burned down.